I'm pretty sure it's implied that Doom & Doom II weren't fully removed from the continuity, though.
IIRC, the original games end with the Doom Marine stuck in hell... Which is also where they found the Doom Slayer. Additionally, despite his background with the Argenta, the Slayer is confirmed to be human in an audio recording; and his Praetor armor is similar enough to the UAC Elite Guard armor that he can upgrade his armor with tokens taken from dead guards.
So... In short, the Slayer might still be the Marine, but with a ton of interstitial backstory that kind of renders his actual origin irrelevant.
A lot of people conflate "knowledge" and "intelligence." Not the guy you replied to, they seem like a troll; but still, a lot of people.
Our ancestors had intelligence in spades. They figured out an insane amount of stuff just to survive; and it's not too far back in the grand scheme of things that they had to remember it all because they had no way to record it. The first caveman to make a handaxe had absolutely no idea what he was doing, but they figured it out. Wheels, bows, fire, the entire concept of agriculture... They figured out how all of that worked from scratch, with no reference material.
Modern humanity builds on that with knowledge. We've figured out how to record everything our ancestors discovered, and all of our new discoveries as well. We've put men on the moon, figured out how to make electricity from things like waterfalls and glowing rocks, and almost everyone has a tiny computer in their pocket.
None of that means that we're more intelligent now, though. All of that knowledge is iterative, so we've just been applying that same intelligence at a continually higher level throughout history.